1. Field of the Invention
This invention is directed to the selective flotation of sulfide minerals from other sulfide minerals. A prime objective is to provide a simple and relatively inexpensive process for practicing selective sulfide flotation using untreated acidic salt-laden water as found in an available source to form the flotation pulp. A specific objective is to provide a simple method for depressing sulfide minerals such as pyrite when using untreated sulfide mine water or untreated water recirculated (recycled) from a sulfide mill so that the to-be-floated sulfide mineral may be concentrated economically.
Flotation plants generally require large amounts of water and the availability of an adequate water supply can be a decisive factor in the selection of a site for a flotation plant. When the plant is to be located in the neighborhood of the mine and an abundant supply of good quality water cannot be conveniently hauled to the plant, it becomes necessary to utilize water available at the plant site. This available water may include so-called "mine water," (as described in SME MINING ENGINEERING HANDBOOK, published by Society of Mining Engineers of the American Institute of Mining, Metallurgical and Petroleum Engineers, Inc. (1973) 26.2-26.6). In some cases it becomes necessary to recirculate water from tailing ponds, thickeners and the like as described in Taggart's HANDBOOK OF ORE DRESSING, published by John Wiley & Sons, Inc. (1927), page 1276.
A considerable number of flotation plants treating sulfide ores must use water originating from mines and/or recirculated water. It is well known that sulfide mine water is usually acidic and contains high concentrations of sulfates of metals such as Fe, Mg, Ca, Na, K, Al, etc. The origin of acidity and an explanation for the occurrence of sulfate salts in mine water appear in the SME hamdbook. In many cases water recirculated from effluents obtained from mills handling sulfide ores is also acidic and salt-laden.
When using mine and/or recirculated plant water for sulfide flotation, selective flotation of a desired metal sulfide mineral value from other sulfides, e.g., iron sulfides, becomes difficult due to the fact that iron sulfides normally float readily in an acid medium when the familiar xanthate sulfide collectors are employed. Such acid pulps therefore limit the possibilities for selective separation of one sulfide from the other. In most cases, the desired concentrate grades can only be achieved by sacrificing the desired sulfide recovery.
One widely used remedy for such systems involves adding heavy dosages of reagents such as lime and sodium cyanide to help depress the not-to-be floated sulfides while avoiding the simultaneous depression of the to-be-floated minerals. Such reagent schedules are relatively expensive and they require great skills on the part of the operators.
Another possible remedy is to process recirculated or mine water by so-called "water treatment" in or prior to flotation. The principal methods for treating mine water and recirculated water are intended to neutralize the acidity and to remove iron. Reference is made to the SME publication. The common and most efficient water treatment makes use of lime and sodium carbonate. Even though this method is relatively frequently put to practice, it requires considerable capital expenditure for new equipment in a water treatment plant as well as additional labor to run such plant.
2. Prior Art
Cyanide salts, including complex zinc-cyanide salts, are known depressant reagents in sulfide flotation. As mentioned, high dosages of lime (to create an alkaline encironment) and sodium cyanide are frequently added to acidic water such as mine water and water from tailing ponds in order to depress iron sulfide minerals. In some flotation operations, both zinc sulfate and sodium cyanide (usually in a ratio of at least 2 parts ZnSO.sub.4 to 1 part NaCN) are used to depress zinc and pyrite while floating lead at an alkaline pH.
More recently a zinc-cyanide complex reagent designated "Reagent S-622" by American Cyanamid Company has been used as a depressant. This reagent is an alkaline solution prepared by reacting about 3.3 moles zinc oxide with 1 mole sodium cyanide in water. Zinc sulfate can also be used to produce the zinc-cyanide complex but when zinc sulfate is employed it must first be neutralized with lime before reaction with sodium cyanide takes place. Zinc-cyanide complexes are used commercially as depressants to separate lead from copper and zinc sulfides and they are also used to separate silver minerals from pyrite. In both cases, the complexes are used with xanthate collectors in pulps that are at alkaline pH values.
U.S. Pat. No. 2,052,214 to Brinker deals with the separation of lead and/or copper sulfide from zinc and/or iron sulfides. The essence of the invention resides in using a zinc cyanide compound, preferably the relatively insoluble zinc cyanide (Zn(CN).sub.2), with a thiocarbanilid reagent to activate the flotation of lead and/or copper sulfides. A soluble lead salt may be added when antimony sulfides are also present in order to activate the antimony, such use of a lead salt activator being disclosed also in U.S. Pat. No. 2,007,176 to Brinker. In carrying out the process disclosed in U.S. Pat. No. 2,052,214, the flotation is carried out in a "nonbasic" pulp. Although the pulp may be acidic or slightly acidic, a neutral pulp is preferred. The U.S. Pat. No. 2,052,214 patent further teaches that the nonpreferred soluble cyanides such as zinc sodium cyanide (Na.sub.2 Zn(CN).sub.4 can be used with some ores but that the soluble cyanides will undesirably "activate zinc and/or iron sulfides." Patentee is not concerned with flotation in acidic, salt-laden pulps. Furthermore, the patent fails to teach, mention or suggest that selection of the nonpreferred soluble cyanide for use in the nonpreferred acidic pulps with a xanthate collector would result in the desirable depression of iron and zinc sulfides.